1. Field of the Invention
The present invention relates to a method of and an apparatus for measuring thickness of an insulation film on the surface of a semiconductor wafer, and more specifically to making the thickness measurement with a non-destructive measuring device.
2. Description of the Related Art
Measurement of the C-V curves is one of known methods used for evaluating semiconductors having an MIS (Metal-Insulator-Semiconductor) structure. In conventional C-V measurement, a test electrode is to be formed on the surface of an insulation film on a semiconductor wafer. Formation of the test electrode, however, affects electric properties of the semiconductor wafer and furthermore takes both labor and time. As a result of extensive study, the inventors of the present invention have developed a novel non-destructive measuring device for evaluating electric properties of semiconductor wafers including C-V and C-t curves without forming any test electrode on the surface of the semiconductor wafers.
FIGS. 1(a) through 1(c) show some features of the non-destructive measuring device for semiconductors developed by the applicants. As shown in FIG. 1(a), an oxide film 102 is formed on a top surface of a semiconductor substrate 101. In measurement of C-V curves, an electrode 202 is electrically connected with a bottom surface of the semiconductor substrate 101, and a sensor head 60 having a test electrode 201 attached to a bottom surface thereof is held above the oxide film 102 via a gap d.sub.air. The gap d.sub.air is adjusted to be not greater than about 1.mu.m.
A total electrostatic capacity C.sub.t between the two electrodes 201 and 202 is defined as a total of a capacity C.sub.si of the semiconductor substrate 101, another capacity C.sub.ox of the oxide film 102, and still another capacity C.sub.air of the gap d.sub.air, which are connected in series. The C-V curve represents voltage dependency of a composite capacity C.sub.ta of the capacity C.sub.si of the semiconductor substrate 101 and the capacity C.sub.ox of the oxide film 102. The gap d.sub.air is first determined precisely based on the reflectance of a laser beam L reflected from the bottom surface of the sensor head 60 under the geometric-optical total reflection conditions. The electrostatic capacity C.sub.air of the gap d.sub.air is then calculated using the gap d.sub.air thus determined. The total capacity C.sub.t is measured, and the composite capacity C.sub.ta is finally obtained dependency of the composite capacity C.sub.ta is thus obtained. by subtracting the capacity C.sub.air of the gap d.sub.air from the total capacity C.sub.t previously measured. A C-V curve, or voltage dependency of the composite capacity C.sub.ta is thus obtained.
The gap d.sub.air is determined from the reflectance of the laser beam L as mentioned above, but precise determination of the gap d.sub.air requires consideration of multi-path reflection at the oxide film 102 on the surface of the semiconductor. Accordingly, for the precise determination of the gap d.sub.air the thickness of the oxide film 102 must be made. In the conventional method, the thickness of the oxide film 102 is measured with a thickness gauge prior to the determination of the gap d.sub.air using the non-destructive measuring device.